A corrupt government has to know they want you "to just get you". Instead they will embed a collision in anti-government meme. That collision will flag you, and now they know you harbor doubts and will come get you.
This is why it's a privacy concern. It's no the tech (like you said photo dna's been about forver), it's the scanning of the phone.
[1]: https://www.pcmag.com/news/fbi-sold-criminals-fake-encrypted...
After all, they want to suppress dissent, and they can't catch and arrest everyone - it's much more effective if people don't even dare to voice their dissent.
This is also why you see people in situations like the Arab Spring use known-insecure/monitored means of communication, because they realize that the value and power that comes from communicating and finding other likeminded people is worth painting a target on yourself (because you can't succeed without it, and if you succeed, there will be too many to prosecute).
Only if that government has worked out some deal with Apple whereby such an anti-government meme would end in the government being notified accordingly. Don't forget that you need a sufficiently high number of collisions, for one, and that those collisions are audited by Apple before being sent to law enforcement.
Not without the risk of it being discovered (either through a leak or because someone analyzes the software on the phone), and then having a much bigger scandal on hand.
You don't even have to implement the feature. All you need is really good proof that you were asked to, and now your job at Apple is secure, along with a huge raise, for years if not for life. Something commensurate with what you and they both know they would lose in the PR damage and possible EU fines.
Until the EU makes it a legal requirement. Which they're getting close to
Also, they'd probably not use a Cupertino developer. I'm sure a dev in a nation with a lot less rights is easier for this sort of work. Find a nation where the protections for employees are worse and good jobs harder to find.
Photos people send me to my Android are automatically sent through 3rd parties, either through MMS, Facebook messenger, Google Photos, or One Drive. Photos arriving on my device are almost guaranteed to be uploaded to both OneDrive and Google Photos based on how defaults of Android phones are setup.
So someone could already send hash collisions my way (purposely or inadvertently) and authoritarian governments already have access in their respective clouds (at least China does).
And yet, there are not stories of people being falsely accused of child porn due to PhotoDNA hash collisions.
Why does "on device for apple devices only" change the calculus.
Everyone suspected they were scanning images for this and a number of other things on their services.